The present invention generally relates to tanks for liquid storage. More particularly, this invention relates to a multiple-walled liquid storage tank equipped with one or more manways located in a wall of the tank.
Various types of liquid storage tanks are employed in industry. A frac tank is a particular example of storage tank developed for the drilling and oil field industries and used in various other fields where containment of a material is important for safety and environmental reasons. Frac tanks typically have liquid capacities in excess of 10,000 gallons (about 40,000 liters), and may be equipped with wheels to facilitate the transport of the tank between job sites. Their large capacities, mobility, and rugged designs make frac tanks well suited for storing a variety of liquids. The sidewalls, end walls, and floors of frac tanks used to store particularly hazardous liquids often have a double-walled construction with interior and exterior walls separated by a closed interstitial cavity. With this construction, failure of the interior wall does not result in leakage outside the tank, but instead results in the accumulation of liquid within the interstitial cavity. The integrity of the interior wall can be monitored by inspecting the interstitial cavity for the presence of liquid.
Inevitably, the interior of a storage tank requires cleaning, often with the result that a worker must enter the interior of the tank through a manway. Conventional constructions of frac tanks and other large storage tanks typically locate manways in the roof of the tank, where a single-walled construction is permitted for most applications. In addition to reducing the risk of leakage through its cover, the location of a manway in the roof of the tank also avoids the complication of providing a fluid-tight manway in a storage tank whose sidewalls have a double-walled construction.